public class Test01 {
    class Solution {
        public int waysToStep(int n) {
            int MOD = (int) 1e9 + 7;

            if (n == 1) {
                return 1;
            } else if (n == 2) {
                return 2;
            } else if (n == 3) {
                return 4;
            }
            //创建dp数组
            int[] dp = new int[n + 1];
            dp[1] = 1;
            dp[2] = 2;
            dp[3] = 4;
            for (int i = 4; i <= n; i++) {
                dp[i] = ((dp[i - 1] + dp[i - 2]) % MOD + dp[i - 3]) % MOD;
            }
            return dp[n];
        }
    }
}
